The internal resistance of a lambda probe is significantly influenced by its temperature and can therefore be applied as a substitute quantity for the probe temperature. The knowledge of the probe temperature is useful for several purposes. The knowledge of the probe temperature permits, inter alia, a diagnosis of the probe heater as is required, for example, by the California Environmental Authority (CARB). Various methods for determining the internal resistance are already known. One known method provides impressing an alternating current signal on the probe which distinguishes from the probe use signal with respect to frequency so that both signal components can be separated by frequency filtering. The amplitude of the alternating-current signal obtained with filtering is dependent upon the temperature of the probe and therefore upon the internal resistance and is therefore suitable as a substitute quantity for temperature.
U.S. Pat. No. 5,140,535 discloses an arrangement for determining the internal resistance of the probe by connecting the positive pole of the probe to ground in a clock pulsed manner via a measurement resistor. The signal is transmitted to a computer in both clocked phases via a preamplifier and the computer computes the internal resistance of the probe from the signal and known values of the circuit.